WASHINGTON — As President Trump moves to fulfill one of the central promises of his campaign — to get tough on an ascendant China — he faces a potential rebellion from a core constituency: farmers and other agricultural producers who could suffer devastating losses in a trade war.

Trump’s threat to impose tariffs on Chinese goods came with a presidential declaration that trade wars are good and easily won. But the action has injected damaging uncertainty into the economy as Republicans are already struggling to maintain their hold on Congress in a difficult election year.

While the battle for control of the House will be waged in large part in the suburbs, rural districts in southern Illinois, Iowa, Arkansas, and Missouri could prove important.

And control of the Senate could come down to Republican efforts to unseat Democrats in North Dakota, Indiana, Missouri, and Montana — all states staring down the barrels of a trade war’s guns.

With farmers angry and worried as China vows to retaliate, many Republicans find themselves torn between loyalty to a president who remains broadly popular in rural states and the demands of constituents, especially farmers, to oppose his tariffs.

In North Dakota, a major soybean-producing state, Representative Kevin Cramer, a Republican who is running for the Senate, sounded restrained this past week when he urged Trump to “take a more measured approach’’ to China. By Friday, he sounded panicked.

He wrote on Twitter that he contacted Agriculture Secretary Sonny Perdue and urged him to use every measure at his disposal to protect farmers from the effects. “Farmers must know the administration has their back and I urge them to act swiftly,’’ he said.

China’s aggressive response to Trump’s tariffs is aimed squarely at products produced in the American heartland, a region that helped send him to the White House. A trade war with China could be particularly devastating to rural economies, especially for pig farmers and soybean and corn growers. Nearly two-thirds of US soybean exports go to China.

The tariffs have not yet gone into effect, and the administration is engaging in back-channel talks with the Chinese to try to resolve their differences.

In the meantime, Trump has been escalating his threats, and shows no sign of backing off. He threatened Thursday to impose tariffs on an additional $100 billion in Chinese products.

Trump administration officials argue that the tariffs on Chinese goods, while not intended to help certain American industries, are necessary to prevent China from continuing to violate international trade rules.

They say that less aggressive measures by past administrations failed, and that China has stolen American jobs and technology that are the key to future prosperity.

“It’s not possible to have true gain without the pain,’’ said Dan DiMicco, a trade adviser to Trump during the presidential campaign. “The battle is worth the victory, and we will win.’’

But farm-state Republicans like Cramer believe that their constituents could be a casualty, and they are begging the Department of Agriculture to intervene.

Trump has directed the department to implement a plan to help farmers cope with the damage from tariffs. But few details have been forthcoming about how such a program would work or how much it might cost.

And it is not clear how much the Agriculture Department could do to remedy the damage done to key trading relations in a global economy.

If the White House does end up subsidizing farmers, that may open trade conflicts on other fronts. Chad P. Bown, a senior fellow at the Peterson Institute for International Economics, said that such a measure could spark trade challenges from countries beyond China, which would object to unfair competition.

Patrick Delaney, a spokesman for the American Soybean Association, said his group was still focused on trying to prevent the tariffs from going into effect, rather than examining what kind of measures are needed to support farmers.

“It’s a whole lot easier not to wreck the car in the first place than it is to think about what a repair might look like,’’ he said.